1. Field of the Invention
This invention relates to a glass substrate for a solar cell having a curved surface suitable for a window glass panel for an automobile, a method for producing the same, and a solar cell. Particularly, it relates to a solar cell-installed window glass panel for an automobile. More particularly, it relates to such window glass panel for an automobile transparent to visible light, particularly suitable as a sunroof, opera window or the like for an automobile.
2. Discussion of the Background
A solar cell is regarded as a prospective photo-electric conversion apparatus which is capable of cleanly producing electric power without causing environmental pollution by using sunlight as an inexhaustible energy source unlike other energy sources such as petroleum, coal or natural gas.
Among such solar cells, a solar cell using amorphous silicon (a-Si) as a semi-conductor layer for effecting photoelectric conversion is widely used since it can be produced at a relatively low cost and it assures a relatively satisfactory photoelectric conversion efficiency. As a typical example of such an amorphous silicon (a-Si) solar cell, FIG. 4 illustrates an a-Si solar cell 48 having an a-Si semiconductor layer 46 composed of p-type a-Si layer 43, i-type a-Si layer 44 and n-type a-Si layer 45 and an aluminum electrode 47 laminated sequentially on a transparent electroconductive film 42 formed on a transparent insulating sheet 41. In the a-Si solar cell 48, light 49 incident on the transparent insulating sheet 41 is absorbed in the a-Si layer 46 thereby generating an electromotive force between the transparent electroconductive film 42 and the aluminum electrode 47, and the electric power thus generated is taken out through lead wires 50.
Small size types of such a-Si solar cells are already used as cells for table computers, watches and the like. In future, such cells are expected to be used for power generation, automobiles, ships, and the like by improving their photoelectric conversion, enlarging their sizes, and lowering their manufacturing costs. In particular, a solar cell for an automobile is expected to be provided in an automobile for use as an electric power supply to drive a ventilator when the automobile is parked or to drive other electronic equipment inside the automobile.
It is expected that a solar cell will be used in the above-mentioned fields. For a practical application, however, a solar cell having a curved surface is desired depending on its particular purpose. For example, an automobile body is desired to have a curved shape in view of aerodynamic efficiency and design, and accordingly a solar cell mounted thereon is also required to have a curved surface fitted thereto.
Heretofore, it is known to use a bilayer glass having a solar cell of a flat plate-like small silicon wafer embedded between two glass plates, as a sunroof and the like. However, there has been a restriction such that the silicon wafer itself can not be subjected to a bending treatment, and it must be used in the state of a flat plate. It is also inferior from the viewpoint of manufacturing costs and productivity. From the viewpoint of the design, productivity and the like, it is desired that a solar cell itself has a curved surface shape.
It is conceivable to use a flexible plastic film as the transparent insulating sheet in order to make a curve-shaped solar cell, but the plastic film is generally inferior in weather-resistance. Furthermore, when it is used for an automobile in the state of being exposed to the atmosphere, it is weak in impact-resistance, scratch-resistance and the like. It is also not resistant to high temperatures at which an a-Si semiconductor film of a solar cell is formed. Thus, a plastic film has been generally unsuitable as the transparent insulating sheet for a solar cell.
As mentioned above, there have been attempts to use a solar cell as an electric power source for driving an air ventilator during parking or other electronic equipments of an automobile. For example, Japanese Unexamined Patent Publication No. 115719/1986 discloses a sunroof for an automobile having a solar cell mounted thereon. However, the solar cell used therein is not visible light-transmissive, but rather it is intended to impart a light-shielding property to the sunroof by using a light-shielding solar cell. Japanese Unexamined Patent Publication No. 199466/1988 discloses a light-transmissive solar cell, but there is no provision for a protective layer.
On the other hand, an automobile having a glass plate panelled sunroof window is popular as the glass-panelled sunroof gives an open-minded feeling to drivers and passengers. Accordingly, a solar cell mounted on the automobile and a protective layer therefor are required to be visible light-transmissive for this purpose. However, if the protective layer is light-transmissive, a grid electrode, a collective busbar and the like of the solar cell are visible from the inside of the automobile, thus spoiling the appearance. Moreover, there has been a problem that when a highly reflective metal such as Ag, Al, Cr and the like is used as a back electrode of the solar cell, drivers and passengers are dazzled by the glare from the highly reflective metal and their eyes are sometimes fatigued.